<template>
  <ul class="list">
    <li
      class="item"
      v-for='item of letters'
      :key='item'
      @click='handleLetterClick'
      @touchstart.prevent='handleTouchStart'
      @touchmove='handleTouchMove'
      @touchend='handleTouchEnd'
      :ref='item'
    >{{item}}</li>
  </ul>
</template>

<script>
export default {
  name: 'CityAlphabet',
  props: {
    cities: Object
  },
  data () {
    return {
      touchStatus: false,
      start: 0,
      timer: null
    }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop
  },
  computed: {
    letters () {
      const letters = []
      for (let i in this.cities) {
        letters.push(i)
      }
      return letters
    }
  },
  methods: {
    // 点击时会传递一个event
    handleLetterClick (event) {
      this.$emit('change', event.target.innerText)
    },
    handleTouchStart () {
      this.touchStatus = true
    },
    handleTouchMove (event) {
      if (this.touchStatus) {
        // 设定一个定时器，函数节流(截留?)，目的是减少函数执行次数，提高性能
        if (this.timer) {
          clearTimeout(this.timer)
        }
        this.timer = setTimeout(() => {
          const touchY = event.touches[0].clientY - 79
          const index = Math.floor((touchY - this.startY) / 20)
          if (index >= 0 && index <= this.letters.length) {
            this.$emit('change', this.letters[index])
          }
        }, 16)
      }
    },
    handleTouchEnd () {
      this.touchStatus = false
    }
  }
}
</script>

<style lang='stylus' scoped>
  @import '~styles/varibles.styl'
  .list
    display flex
    flex-direction column
    justify-content center
    position absolute
    top 1.58rem
    right 0
    bottom 0
    width .4rem
    .item
      text-align center
      line-height .4rem
      color $bgColor
</style>
